1. Field of the Invention
The present invention relates to an oil pressure control system for an automatic transmission system for use with automobiles.
2. Description of the Prior Art
In an automatic transmission system which includes a fluid torque converter, a transmission gear and friction engaging means for establishing a selected transmission engagement in said transmission gear, said automatic transmission system being controlled by an oil pressure control system, the manner of actuating said friction engaging means is timely altered for automatically establishing the most favourable transmission of power by said transmission gear according to the operating condition of the vehicle or for establishing a particular transmission as determined by the driver. If engagement and disengagement of said friction engaging means are abruptly effected for the above mentioned variation in the engagement combination of said friction engaging means, a shock is given to the vehicle. This shock does not provide a comfortable ride and damages the transmission gear and other power transmission mechanisms. As a means for avoiding generation of shock in engagement and disengagement of the friction engaging means, an orifice-type control valve is known, which is adapted to change the number of orifices according to the vehicle speed. However, the orifice-type control valve can only regulate oil pressure supplied to a particular friction engaging means without keeping any timing relation with respect to the operation of other friction engaging means. Therefore, it is very difficult to accomplish proper overlapping in operation of two friction engaging means, one being engaged while the other being disengaged.